Color electrophoretic display device

ABSTRACT

An electrophoretic display device includes a substrate and unit cells. Each of the unit cells has one display electrode, two collection electrodes and two types of charged particles different from each other in charged polarity and coloration, and has a driving means capable of forming a state in which each of the two types of charged particles have been collected respectively at each of the collection electrodes, a state in which one of the two types of charged particles have been disposed on the display electrode and the other type of charged particles have been collected at one of the collection electrodes, a state in which one of the two types of charged particles have been collected at one of the collection electrodes and the other type of charged particles have been disposed on the display electrode, and a state intermediate between these states. A color display device is provided which promises bright and sharp-color display and has a display quality level close to that of hard-copy representation mediums.

RELATED APPLICATION INFORMATION

This application is a continuation of U.S. application Ser. No.10/794,121 filed Mar. 4, 2004 now U.S. Pat. No. 7,227,525, which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a color display system in electrophoreticdisplay devices.

2. Related Background Art

In recent years, with advancement of information machinery, the quantityof data of various information is becoming larger and larger, and theinformation is also outputted in various forms. The outputting ofinformation is commonly roughly grouped into “display representation”making use of a cathode-ray tube, a liquid-crystal display panel or thelike and “hard-copy representation” on paper by means of a printer orthe like.

In the display representation, there is an increasing need for displaydevices of low power consumption and small thickness. In particular,liquid-crystal display devices have energetically been developed andcommercialized as display devices adapted for such need. Inliquid-crystal display devices available at present, however, charactersor letters displayed on a screen may be viewed with difficulty dependingon the angles at which the screen is viewed or under the influence ofreflected light, and the load on the eyes which is caused by flickering,low luminance and so forth of a light source has not been properlysolved. Also, in the display representation making use of a cathode-raytube, although it provides sufficient contrast and luminance comparedwith the liquid-crystal display, it may cause flickering, and can not besaid to have a sufficient display quality level compared with thehard-copy representation discussed below. In addition, its display unitsare so large and heavy as to have a very low portability.

Meanwhile, the hard-copy representation has been considered to becomeunnecessary as information is made electronic, but in fact hard copiesare still taken in vast quantities. As reasons therefor, the followingcan be given. In the case of display representation of information inaddition to the above problem concerning the display quality level, thedisplay has a resolution of 120 dpi at maximum, which is fairly lowerthan that of prints on paper (usually 300 dpi or higher). Hence, thedisplay representation may greatly task the eyes compared with thehard-copy representation. As a result, even though the information canbe viewed on a display, it is first outputted on a hard copy. Inaddition, the information represented on hard copies can be arranged ina large number of sheets without any limitation of the display area asin the display representation, can be rearranged without any complicatedmachine operation, or can be checked in order. These are also largereasons why the hard-copy representation is used in combination eventhough the display representation is feasible. Furthermore, thehard-copy representation does not require any energy for retaining itsrepresentation, and has a superior portability so that the informationcan be checked anytime and anywhere as long as the information is not soextremely much.

Thus, as long as any motion-picture display or frequent rewriting is notrequired, the hard-copy representation has various advantages differentfrom the display representation, but has such a disadvantage that paperis consumed in a large quantity. Accordingly, in recent years,development is energetically put forward on a rewritable recordingmedium (a recording medium on which highly visible images can repeatedlybe recorded and erased in many cycles and which does not require anyenergy for retaining its representation). The third way ofrepresentation which has succeeded the features the hard copies have andin which images are rewritable is herein refered to as “paper-likedisplay”.

Requirements for the paper-like display are such that images arerewritable, that energy for retaining the display is not required or issufficiently low (memory performance), that the display has a goodportability, that the display has a good quality level, and so forth. Atpresent, as a representation method which can be regarded as thepaper-like display, for example, a reversible display medium isavailable using an organic low-molecular and high-molecular resin matrixsystem which is recorded and erased with a thermal printer head (e.g.,Japanese Patent Applications Laid-Open No. S55-154198 and No.S57-82086). This system is partly utilized as a display area of aprepaid card, but has problems such that the contrast is not so high andthe writing and erasing can only be repeated as small as 150 to 500times.

As a way of display which is expected to be utilized as anotherpaper-like display, an electrophoretic display device invented by HaroldD. Lees et al. (U.S. Pat. No. 3,612,758) is known. Besides, JapanesePatent Application Laid-Open No. H9-185087 discloses an electrophoreticdisplay device.

This display device is constituted of a dispersion medium having aninsulating liquid in which colored charged particles stand dispersed,and a pair of electrodes which are set face to face holding thisdispersion medium between them. It is a device in which, uponapplication of a voltage to the dispersion medium via the electrodes,the colored charged particles are attracted by Coulomb force to theelectrode side having a polarity reverse to that of electric charges theparticles themselves have, by utilizing electrophoretic properties ofthe colored charged particles. Its display is performed utilizingdifferences between the color of the colored charged particles and thecolor of an insulating liquid having been dyed. That is, the color ofthe colored charged particles is perceived when the colored chargedparticles are kept attracted to the surface of a first electrode near tothe observer side and having light transmission properties. On thecontrary, when the colored charged particles are kept attracted to thesurface of a second electrode distant from the observer side, the colorof the insulating liquid having been dyed is perceived, which has beenso dyed as to have optical characteristics different from those of thecolored charged particles.

However, in such an electrophoretic display device (hereinafter oftenrefered to as “vertical-movement type electrophoretic display device”),a coloring material such as a dye or ions must be mixed in theinsulating liquid, and the presence of such a coloring material tends toact as an unstable factor in electrophoretic movement because it bringsabout the delivering and receiving of additional electric charges,resulting in a lowering of performance, lifetime and stability as adisplay device in some cases.

In order to solve such a problem, an electrophoretic display device inwhich a pair of electrodes consisting of a first display electrode and asecond display electrode are disposed on the same substrate and thecharged particles are moved horizontally as viewed from the observerside, has been proposed as disclosed in Japanese Patent ApplicationsLaid-Open No. S49-5598 and No. H10-005727. It is a device in which,utilizing electrophoretic properties of colored charged particles,display is performed by making the colored charged particles movehorizontally to the substrate surface between the surface of the firstdisplay electrode and the surface of the second display electrode in atransparent insulating liquid by applying a voltage.

In such a horizontal-movement type electrophoretic display device, theinsulating liquid is transparent in many cases. As viewed from theobserver side, the first display electrode and the second displayelectrode are differently colored, and either of their colors has beenmade to have the same color as the colored charged particles. Forexample, where the color of the first display electrode is black, thecolor of the second display electrode is white and the color of thecolored charged particles is black, the second display electrode comesuncovered to see white when the colored charged particles standdistributed over the first display electrode, and see black as the colorof the colored charged particles when the colored charged particlesstand distributed over the second display electrode.

Now, the most fundamental system for materializing color display in theabove electrophoretic display devices is a system in which three unitcells respectively having the three primary colors consisting of RGB orYMC are disposed in parallel on the same plane to make up each pixel andthe color display is performed by the principle of additive colormixing. In either system of the vertical-movement type and thehorizontal-movement type, each unit cell has one type of colored chargedparticles, two drive electrodes and a colored migration liquid, wheretwo colors, the color of the colored charged particles and the color ofthe colored migration liquid, or the color of the colored chargedparticles and the color of a color filter, can be shown by the movementof the particles.

For example, in Japanese Patent Applications Laid-Open No. 2000-035589,three unit cells having different colored liquids with the three primarycolors are disposed in parallel to form each pixel. Unit cells formed ofmicrocapsules in which a colored liquid and white particles have beenenclosed are ejected from nozzles so that microcapsules having differentcolored liquids (migration liquids) with the three primary colors,yellow (Y), cyan (C) and magenta (M) are regularly arranged. Eachmicrocapsule changes alternately in two colors, the white which is thecolor of the particles and the color of the migration liquid, by thevertical movement of the white particles.

Also in the case of the horizontal-movement type, three unit cellsshowing different colors for color display are arranged to make up eachpixel. Each unit cell is filled with a transparent insulating liquid andblack particles. On the display electrode surfaces of the unit cells,different color filters with the three primary colors, red (R), green(G) and blue (B), are respectively disposed in order from the left cell.Each unit cell changes alternately in two colors, the black which is thecolor of the particles and the color of each color filter, by thehorizontal movement of the black particles.

In any of the above systems, when color display is performed, each pixelis formed by the three unit cells disposed adjoiningly and having colorscorresponding to the three primary colors, and the desired display coloris formed by the principle of additive color mixing.

However, in the additive color mixing of the three primary colors, it istheoretically impossible to achieve brightness and color sharpness(inclusive of sufficient black display) simultaneously, and it is verydifficult to materialize a reflection type display device having thedisplay quality level the printed mediums can have. In the case of theadditive color mixing by the use of white particles plus the threeprimary colors Y, C and M, a satisfactory level can be achieved inrespect of the brightness, but colors of pastel shades lacking in colorsharpness are shown because the white light component is alwayssuperimposed on the background of reflected light, and also anysufficient black is not obtainable. A sufficient black is obtainable ifblack particles are used, but such a measure is insufficient in respectof the brightness and the color sharpness. On the other hand, in thecase of the additive color mixing by the use of black particles plus thethree primary colors R, G and B, the intensity ratio of reflected lightto incident light is 1/9 or less in the monochrome display and ⅓ or lessin the white display, where any sufficient brightness is not achieved.The brightness is improved if white particles are used, but any sharpcolor representation is not obtainable and also any sufficient black isnot obtainable.

In WO 99/53373, in order to improve brightness and sharpness of colorsin the additive color mixing, a structure is disclosed in which unitcell microcapsules change in three colors. Three unit cells showingdifferent colors for color display are arranged to make up each pixel.In this structure, which is called “dual particle curtain mode”, themicrocapsules are filled with an migration liquid in which two types ofcolored charged particles having different charge polarities and colorshave been dispersed. By applying voltage to three drive electrodesdisposed in the unit cells, the two types of colored charged particlesare moved independently, where each unit cell can be changed alternatelyin three colors, the three colors of the two types of colored chargedparticles and the color of the migration liquid, or the three colors ofthe two types of colored charged particles and the color of each colorfilter disposed on the back of the microcapsules. A typical cellstructure disclosed in this WO 99/53373 is shown in FIGS. 17 to 18D. Adisplay electrode is disposed on the side of a front substrate, and twocollection electrodes to which different voltages can be applied aredisposed on the side of a back substrate. Its insulating liquid istransparent, and a color filter is disposed on the back of each unitcell. For example, in the case of a combination of white chargedparticles with the electrophretic particles and a color filter standingin a complementary color relation to the charged particles, thesharpness and brightness of colors can be improved in respect ofmonochrome display (FIG. 18B) and complementary-color display (FIG.18C).

In the structure shown in FIGS. 17 to 18D, however, the collectionelectrodes are both formed on the back substrate side, and hence, theopen-area ratio, which is of the area in which the color filter disposedon the back of each unit cell functions effectively (the display areashown in FIG. 17), is remarkably reduced inevitably. As a result, thereis a problem in that the sharpness and brightness of colors that areoriginally to be aimed at are insufficient.

Use of not the color filter but the colored insulating liquid enablesavoidance of the above problem to a certain extent. In such a case,however, there is a problem in that a coloring dye added to theinsulating liquid tends to cause deterioration in extensive operationdue to electrode reaction and contrast deterioration due to the dyeingof charged particles.

There also is a problem in that since the number of electrodes becomelarger by one from the conventional two electrodes to the threeelectrodes, this consequently makes it necessary to narrow wiringpitches at a display medium panel portion or causes a rise in drive ICcosts.

SUMMARY OF THE INVENTION

Taking into account the problems discussed above, an object of thepresent invention is to provide a color electrophoretic display device,in particular, a reflection type color electrophoretic display device,having been improved in brightness and color sharpness and havingachieved a display quality level closer to that of hard-copyrepresentation mediums.

The object of the present invention can be achieved by the followingmeans.

That is, the present invention provides an electrophoretic displaydevice comprising unit cells each of which has one display electrode,two collection electrodes and two types of charged particles which aredifferent from each other in charge polarity and coloration, and has adriving means capable of forming a state in which each of the two typesof parged particles have been collected at each of the collectionelectrodes, a state in which one of the two types of charged particleshave been disposed on the display electrode and the other type ofcharged particles has been collected at one of the collectionelectrodes, a state in which one of the two types of charged particleshave been collected at one of the collection electrodes and the othertype of charged particles have been disposed on the display electrode,and a state intermediate between these states.

As an invention to remedy the loss of open-area ratio due to the twocollection electrodes, there is proposed the constitution in which thetwo collection electrodes are disposed at a position where they liesubstantially one upon another as viewed from the observer side.

Stated more specifically, proposed are:

the constitution in which a partition wall is provided which dividesadjoining unit cells, and the two collection electrodes are disposed onboth ends of the partition wall or in the interior of the partitionwall; or

the constitution in which at least one of the two collection electrodesis disposed in a recessed structure such as a hole or a groove in whichthe charged particles are holdable; and

the constitution in which the unit cell has a partition wall whichdivides adjoining unit cells, and at least one of the two collectionelectrodes is disposed at the bottom of a groove formed adjoiningly tothe partition wall and around the unit cell.

As another invention to remedy the loss of open-area ratio, there isproposed the constitution in which the unit cell has a partition wallwhich divides adjoining unit cells, and at least one of the twocollection electrodes is disposed on the surface of, or in the interiorof, the partition wall.

As specific constitution of a color electrophoretic display device,proposed is:

the constitution in which pixels in each of which three unit cells areadjoiningly disposed in parallel are arranged in matrix form, the twotypes of charged particles contained in the three unit cells are allblack particles and white particles, and color filters with the threeprimary colors red, green and blue, or cyan, magenta and yellow, arerespectively disposed on the back sides of the three unit cells; or

the constitution in which pixels in each of which three unit cells areadjoiningly disposed in parallel are arranged in a matrix, the colors ofthe two types of charged particles in the three unit cells are incombination of red and green, green and blue, and blue and red,respectively, and white scattering layers are disposed on all the backsides of the three unit cells; or

the constitution in which pixels in each of which three unit cells areadjoiningly disposed in parallel are arranged in a matrix, the colors ofthe two types of charged particles in the three unit cells are incombination of red and cyan, green and magenta, and blue and yellow,respectively, and white scattering layers are disposed on all the backsides of the three unit cells; or

the constitution in which pixels in each of which three unit cells areadjoiningly disposed in parallel are arranged in a matrix, the color ofone of the two types of charged particles contained in the three unitcells is white, color filters are disposed on the back sides of thethree unit cells, and the color of the other charged particles containedin the three unit cells and the colors of the color filters disposed onthe back sides are in combination of red and cyan, green and magenta,and blue and yellow, respectively; or

in the right above constitution, the constitution in which at least oneof the two types of charged particles is light-transmissive coloredparticles, and display colors are formed by the principle of subtractivecolor mixing when the light-transmissive colored particles are arrangedon the color filters.

The constitution is also proposed in which the unit cell contains amicrocapsule enclosing an migration liquid in which the two types ofcharged particles stand dispersed.

As an invention to make it unnecessary to narrow wiring pitches at adisplay medium panel portion, or reduce a rise in drive IC costs,proposed are:

in an electrophoretic display device in which pixels comprising unitcells are arranged in matrix foprm, the constitution in which the twocollection electrodes in each unit cell are common electrodes to eachother to which a voltage common to all the pixels is to be applied; and

a method for driving an electrophoretic display device in which pixelscomprising unit cells are arranged in matrix form; the methodcomprising:

a first step of applying a drive pulse common to all the pixels to eachof one display electrode and two collection electrodes in each unit cellto collect charged particles to the collection electrodes; and

a second step of applying a drive pulse common to all the pixels to eachof the two collection electrodes, and applying a desired different drivepulse corresponding to image information for each pixel, to the displayelectrode to perform writing.

As summarized above, in the present invention, in the electrophoreticdisplay device having one display electrode, two collection electrodesand two types of charged particles showing charge polarity and colordevelopment which are different from each other, the two collectionelectrodes are disposed at a position where they lie substantially oneupon another as viewed from the observer side, or disposed at the sidesof partition walls. Such constitution can vastly remedy the loss ofdisplay area open-area ratio that is due to the introduction of the twocollection electrodes.

A driving method is also proposed which makes it possible to set the twocollection electrodes as common electrodes by a novel drive sequence,thereby avoiding the restriction on electrode pitches and the rise indrive IC costs that are due to signal lines formed in a larger number.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are sectional views showing the concept of a unit cell.

FIGS. 2A, 2B and 2C are sectional and plan views showing variations ofthe structure of the unit cell.

FIGS. 3A and 3B are sectional views showing variations of the structureof the unit cell.

FIG. 4 is a sectional view and a plan view showing a variation of thestructure of the unit cell.

FIGS. 5A and 5B are sectional and plan views showing variations of thestructure of the unit cell.

FIGS. 6A, 6B and 6C are sectional views showing variations of thestructure of the unit cell.

FIGS. 7A, 7B, 7C, 7D, 7E and 7F illustrate a method of driving the unitcell.

FIGS. 8A, 8B, 8C, 8D, 8E and 8F illustrate another method of driving theunit cell.

FIG. 9 is a sectional view of a pixel structure of aparallel-disposition type in its first example.

FIGS. 10A, 10B, 10C and 10D illustrate a color display method in thefirst example of the pixel structure of a parallel-disposition type.

FIG. 11 is a sectional view of the pixel structure of aparallel-disposition type in its second example.

FIGS. 12A, 12B, 12C and 12D illustrate a color display method in thesecond example of the pixel structure of a parallel-disposition type.

FIG. 13 is a sectional view of the pixel structure of aparallel-disposition type in its third example.

FIGS. 14A, 14B, 14C and 14D illustrate a color display method in thethird example of the pixel structure of a parallel-disposition type.

FIGS. 15A and 15B are structural views in Example.

FIGS. 16A, 16B and 16C illustrate a cell fabrication process in Example.

FIG. 17 is a sectional structural view of a unit cell of the prior art(dual particle curtain mode).

FIGS. 18A, 18B, 18C and 18D illustrate a color display method in theprior art (dual particle curtain mode).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is described below in greater detail withreference to the accompanying drawings.

(Basic Structure of Unit Cell)

FIGS. 1A and 1B are schematic views showing an example of the sectionalstructure of a unit cell which is a basic component of the presentinvention. The space between a back substrate 102 and an observationside substrate 101 which are disposed face to face leaving a constantspace between them is filled with a transparent insulating migrationliquid 5. In this liquid, two types of charged particles 41 and 42different in charge polarity and coloration are dispersed. A displayelectrode 2 is disposed either on the observation side substrate (FIG.1A) or on the back substrate (FIG. 1B) to form a display area. Thedisplay electrode 2 is transparent in the visible light region at leastwhere it is disposed on the observation side substrate.

Two collection electrodes are disposed in the unit cell. One ofcharacteristic features of the present invention in regard to thedisposition of collection electrodes is such that a first collectionelectrode 31 and a second collection electrode 32 are face to facedisposed on the observation side substrate 101 and the back substrate102, respectively, at a position where they lie substantially one uponanother as viewed from the observer side. In order to prevent colorsfrom mixing, the areas where the collection electrodes are disposed maypreferably be light-screened in black. A black electrode material mayalso be used, or a black insulating layer may additionally be provided.The loss of open-area ratio of the display area due to the collectionelectrodes can be halved compared with the case in which two collectionelectrodes are disposed only on either of the back substrate and theobservation side substrate.

The position of the two types of collection electrodes is by no meanslimited to the disposition shown in FIGS. 1A and 1B. The following casesmay be conceived: a case in which the collection electrodes 31 and 32are disposed at the middle of the observation side substrate and at themiddle of the unit cell back substrate, respectively (FIG. 2A); a casein which the collection electrodes 31 and 32 are disposed at theperiphery of the observation side substrate and at the periphery of theunit cell back substrate, respectively (FIG. 2B); and a case in whichthe collection electrodes 31 and 32 are disposed on unit cell partitionwall surfaces on the side of the observation side substrate and on theside of the unit cell back substrate, respectively (FIG. 2C).

The collection electrodes may be disposed at the bottoms of recessedportions as shown in FIGS. 3A and 3B. In this case, there is such anadvantage that the charged particles that can be held on the collectionelectrodes in a larger number to enhance the open-area ratio. In thecase shown in FIG. 3A, the two collection electrodes 31 and 32 aredisposed at the bottoms of square hollows made in the observation sidesubstrate and in the back substrate, respectively, at their middleportions. In the case shown in FIG. 3B, the two collection electrodes 31and 32 are disposed at the bottoms of grooves made in the observationside substrate and in the back substrate, respectively, at theirperipheral portions.

The collection electrodes may be disposed at the positions of partitionwalls which divide unit cells, i.e., at the tops and bottoms of eachpartition wall or in the interiors of the partition walls. In such acase, the collection electrodes disappear from the sight inside thepixels, and hence the open-area ratio can be expected to be vastlyenhanced. In the structure shown in FIG. 4, the two collectionelectrodes 31 and 32 are disposed at the tops and bottoms, respectively,of partition walls 11.

One of other characteristic features in regard to the disposition ofcollection electrodes in the present invention is in that the collectionelectrodes are disposed at the sides of partition walls (see FIGS. 5A,5B and FIG. 2C). This includes a structure in which the collectionelectrodes are embedded in the interior of partition walls or thepartition walls themselves serve as the collection electrodes. This ismore advantageous in respect of the open-area ratio of display areasthan a case in which the two collection, electrodes are disposed only oneither of the back substrate and the observation side substrate.

Encapsulization (enclosure in microcapsules) of the charged particles inunit cells is also one of preferred embodiments. As shown in FIGS. 6A to6C, each transparent microcapsule 11 having enclosed two types ofparticles and an migration liquid is disposed in the state it has beenmade flat. FIG. 6A shows an example in which, between substrates havingthe electrode structure shown in FIG. 2B, the air space between bothelectrodes and the microcapsule 11 is filled with a transparent resinbinder 12, in the state of which the microcapsule is pressed with boththe substrates to make it flat, followed by curing of the resin binder12 to fix the microcapsule in a flat shape. As the resin binder, it ispreferable to use an ultraviolet-curable resin or a heat-curable resin.FIG. 6B shows an example in which the microcapsule is fixed in the samemanner but between substrates having the electrode structure shown inFIG. 5A. FIG. 6C shows an example in which the observation sidesubstrate has been omitted in the electrode structure shown in FIG. 2B.The top surface of the resin binder with which the microcapsule is fixedmay be made flat and the electrode and an insulating layer may directlybe formed on the flatted surface, provided that the electrode is formedby a low-temperature process which does not require any vacuumtreatment, such as the printing of organic conductive films.

In the foregoing structures, the number of the first collectionelectrode 31 and second collection electrode 32 are by no means limitedto one for each. A plurality of electrodes may be provided on the backsubstrate and/or the observation side substrate 101.

There are no particular limitations on the plane shape of pixels, whichmay include any shapes as exemplified by polygons such as squares,rectangles and hexagons, and circles.

(How to Drive Unit Cell)

A basic method of driving the unit cells in the present invention isdescribed with reference to FIGS. 7A to 7F. In the present invention,the driving method is characterized in that display in three states anddisplay intermediate between these states are performed by forming afirst state in which the two types of charged particles are bothattracted to the two collection electrodes (FIG. 7B), a second state inwhich only first particles of the two types of charged particles areattracted to the display electrode (FIG. 7C), a third state in whichonly second charged particles are attracted to the display electrode(FIG. 7D), and a state intermediate between these three states.

The respective charged particles are polymer particles colored indesired colors. In these drawings, the first charged particles of thetwo types of charged particles are assumed to be negatively charged andthe second charged particles to be positively charged.

Before the display is performed, it is reset to a initial state (FIG.7A). A continuous rectangular wave voltage of 10 Hz and +20 and −20 V isapplied to the first collection electrode and the second collectionelectrode and another continuous rectangular wave voltage of reversedphaseis applied to the display electrode. Thus, an initial state isformed in which the two types of charged particles stand disperseduniformly in the cell.

To form the first state (FIG. 7B), the display is first reset to theinitial state, and then, keeping the display electrode at 0 V, arectangular pulse of +20 V and 100 ms is applied to the first collectionelectrode, and a rectangular pulse of −20 V and 100 ms to the secondcollection electrode. Most of the first negatively charged paticlescollect to the first collection electrode, and on the other hand most ofthe second positively charged particles collect to the second collectionelectrode, so that the transparent display electrode surface is exposedto the observer side.

To form the second state (FIG. 7C), after the display has been reset tothe initial state, a rectangular pulse of +20 V and 100 ms is applied tothe display electrode, and a rectangular pulse of −20 V and 100 ms tothe second collection electrode while keeping the first collectionelectrode at 0 V. Most of the second positively charged particlescollect to the second collection electrode disposed at thelight-screening area, and on the other hand the first negatively chargedparticles form a particle layer on the display electrode which affords adisplay area.

To form the third state (FIG. 7D), after the display has been reset tobe initialized, keeping the second collection electrode at 0 V, arectangular pulse of −20 V and 100 ms is applied to the displayelectrode, and a rectangular pulse of +20 V and 100 ms to the firstcollection electrode. The first negatively charged particles collect tothe first collection electrode disposed at the light-screening area, andon the other hand the second positively charged particles form aparticle layer on the display electrode which affords a display area.

The state intermediate between these first to fourth states can also beformed (FIGS. 7E and 7F). For example, in the case when an intermediatestate shown in FIG. 7E is to be formed, after the display has been resetto the initial state, a rectangular pulse of +10 V and 100 ms is appliedto the first display electrode, and a rectangular pulse of +20 V and 100ms to the second collection electrode while keeping the first collectionelectrode at 0 V. Some of the first charged particles collect on thefirst collection electrode, and the remaining first charged particlescome dispersed on the display electrode. On the other hand, the secondcharged particles collect on the second collection electrode.

The initial reset state is not necessarily limited to theparticle-dispersed state. For example, the first state (FIG. 6B) may beset as the initial reset state. It is a matter of course that theconditions for voltage application to each state change depending on thereset state.

In the foregoing description, in order to simply describe the basicdriving method, the voltages applied to the display electrode and twocollection electrodes at the time of writing are so changed as to be 0V,+20V and −20 V for each display state as shown in FIGS. 7B to 7D.However, where unit cells are actually arranged in a matrix form anddriven, signal lines must be provided in a larger number than inconventional systems to cause the restriction on electrode pitches andthe rise in drive IC costs.

Accordingly, in the present invention, in order to reduce the number ofelectrodes which must be controlled independently for each pixel, adriving method is proposed in which the two of the three electrodes canbe set to be common electrodes to which the same voltage may be appliedin all pixels. Such a driving method characteristic of the presentinvention is described with reference to FIGS. 8A to 8F.

In FIGS. 8A to 8F, the first collection electrode and second collectionelectrode are set to be common electrodes. First, the display is broughtto the initial reset state to form a first state (FIG. 8A), and then allthe electrodes are set to 0 V. In the writing, in the state the commonelectrodes, two collection electrodes, are fixed to 0V, a desiredvoltage is applied only to the display electrode, whereby a first stateto a third state (FIGS. 8B to 8E) and an intermediate state of these(FIG. 8F) can be formed.

(Pixel Structure and Display Method for Color Display)

A typical pixel structure in the present invention and its color displaymethod are described with reference to FIG. 9 and FIGS. 10A to 10D. FIG.9 shows a schematic cross-section of the pixel structure. The unit cellaccording to the present invention, having been described with referenceto FIG. 1, is used in three which are disposed in parallel to make upeach pixel. In the following, these are refered to as a first cell, asecond cell and a third cell in order from the left-side cell. A firstcollection electrode 31 and a second collection electrode 32 aredisposed on the side of the observation side substrate on the side ofthe back substrate side, respectively, in each cell. The first displayelectrode 21 and second display electrode 22 are disposed at a positionwhere they lie substantially one upon another as viewed from theobserver side.

In what is shown in this drawing, the first cell to third cell are allfilled with insulating liquids in which black positively chargedparticles 47 and white negatively charged particles 48 respectivelystand dispersed. Three-primary-color scattering layers of red, green andblue or cyan, magenta and yellow are respectively disposed on the backsof the respective cells. In what is shown in this drawing, a cyanscattering layer 64 is disposed on the back of the first cell, a magentascattering layer 65 on the back of the second cell, and a yellowscattering layer 66 on the back of the third cell. In this embodiment,the brightness and sharpness of color display are not improved, butthere is such an advantage that the brightness of white display and thelight-screening performance of black display are vastly improved.

A color display method in the pixel structure shown in FIG. 9 isdescribed below with reference to FIGS. 10A to 10D, regarding respectivecases of white display, monochrome display, complementary-color displayand black display.

In the case of white display, as shown in FIG. 10A, in all cells, whiteparticles are arranged on all the display electrodes and black particlesare collected at the second collection electrodes. White incident lightis scattered at white particle layers, and white light is emittedwithout being modulated.

As an example of monochrome display such as red display, green displayor blue display, a case of green display is shown in FIG. 10B. In thefirst cell and third cell, white particles and black particles arecollected at the first collection electrodes and second collectionelectrodes, respectively, to make the cyan scattering layer and theyellow scattering layer uncovered. Also, in the second cell, whiteparticles are collected at the first collection electrode, and a blackparticle layer is formed on the display electrode to hide the magentascattering layer. White incident light is modulated to green light bythe additive color mixing between the cyan light component scattered inthe first cell and the yellow light component scattered in the thirdcell, and the green light is emitted.

As an example of complementary-color display such as cyan display,magenta display or yellow display, a case of magenta display is shown inFIG. 10C. In the first cell and third cell, white particles arecollected at the first collection electrodes, and black particle layersare formed on the display electrodes to hide the cyan scattering layerand the yellow scattering layer. Also, in the second cell, whiteparticles and black particles are collected at the first collectionelectrode and second collection electrode, respectively, to make themagenta scattering layer uncovered. White incident light is modulated tomagenta light scattered in the second cell, and the magenta light isemitted.

In the case of black display, as shown in FIG. 10D, white particles arecollected at the first collection electrodes in all cells, and blackparticle layers are formed on the display electrodes to hide the colorscattering layers. White incident light is all absorbed at the blackparticle layers.

Next, another typical pixel structure in the present invention and itscolor display method are described with reference to FIG. 11 and FIGS.12A to 12D. FIG. 11 shows a schematic cross-section of the pixelstructure. Description concerning the same constituents as shown in FIG.9 is omitted. In this embodiment, different color charged particles aredisposed in cells in different combination for each cell. As thecombination of the colors of charged particles, it may includecombinations of red particles and cyan particles, green particles andmagenta particles, and blue particles and yellow particles; combinationsof red particles and green particles, green particles and blueparticles, and blue particles and red particles; and combinations ofcyan particles and magenta particles, magenta particles and yellowparticles, and yellow particles and cyan particles.

In FIG. 11, the first cell, second cell and third cell are filled withinsulating liquids in which red particles 47 and green particles 48,green particles 48 and blue particles 49, and blue particles 49 and redparticles 47, respectively, stand dispersed. White scattering layers 10are respectively disposed on the backs of the respective cells. In thisembodiment, black display can not be expected to be improved, but thereis such an advantage that the brightness of white display and thebrightness and sharpness of color display are vastly improved.

A color display method in the pixel structure shown in FIG. 11 isdescribed below with reference to FIGS. 12A to 12D, regarding respectivecases of white display, monochrome display, complementary-color displayand black display.

In the case of white display, as shown in FIG. 12A, in all cells, twotypes of color particles are collected at the first collectionelectrodes and second collection electrodes to make the white scatteringlayers uncovered. White incident light is scattered at white particlelayers, and is emitted without being modulated.

As an example of monochrome display such as red display, green displayor blue display, a case of green display is shown in FIG. 12B. In thefirst cell, red particles are collected at the second collectionelectrode, and meanwhile a green particle layer is formed on the displayelectrode. In the second cell, blue particles are collected at the firstcollection electrode, and on the other hand, a green particle layer isformed on the display electrode. Also, in the third cell, blue particlesand red particles are collected at the second collection electrode andfirst collection electrode, respectively, to make the white scatteringlayer uncovered. White incident light is modulated to green light by theadditive color mixing between the green light component scattered in thefirst cell and second cell and the white light component scattered inthe third cell, and is emitted.

As an example of complementary-color display such as cyan display,magenta display or yellow display, a case of magenta display is shown inFIG. 12C. In the first cell, green particles are collected at the firstcollection electrode, and meanwhile a red particle layer is formed onthe display electrode. In the second cell, blue particles and greenparticles are collected at the first collection electrode and secondcollection electrode, respectively, to make the white scattering layeruncovered. In the third cell, red particles are collected at the firstcollection electrode, and meanwhile a blue particle layer is formed onthe display electrode. White incident light is modulated to magentalight by the additive color mixing between the red light componentscattered in the first cell, the white light component scattered in thesecond cell and the blue light component scattered in the third cell,and is emitted.

In the case of black display, as shown in FIG. 12D, in the first cell,green particles are collected at the first collection electrode, and onthe other hand, a red particle layer is formed on the display electrode.In the second cell, blue particles are collected at the first collectionelectrode, and meanwhile a green particle layer is formed on the displayelectrode. In the third cell, red particles are collected at the firstcollection electrode and meanwhile a blue particle layer is formed onthe display electrode. White incident light is modulated to white lightattenuated to ⅓ in intensity, by the additive color mixing between thered light component scattered in the first cell, the green lightcomponent scattered in the second cell and the blue light componentscattered in the third cell, and is emitted.

Next, still another typical pixel structure in the present invention andits color display method are described with reference to FIG. 13 andFIGS. 14A to 14D. FIG. 13 shows a schematic cross-section of the pixelstructure. Description concerning the same constituents as shown in FIG.9 is omitted. In this embodiment, white charged particles, color chargedparticles, and a color reflection layer different in color from thecolor charged particles are introduced into each cell. As thecombination of the colors of color charged particles and the colorreflection layer, it may include combinations of red and cyan, green andmagenta, and blue and yellow; combinations of red and green, green andblue, and blue and red; and combinations of cyan and magenta, magentaand yellow, and yellow and cyan. The color particles may belight-transmissive. In the case where the color particles arelight-transmissive, upon forming color particle layers in the displayareas, white incident light is modulated by the additive color mixingbetween the color particle layers and the color reflection layers.

In FIG. 13, the first cell, second cell and third cell are filled withinsulating liquids in which light-transmissive cyan particles 43 andwhite particles 40, light-transmissive magenta particles 44 and whiteparticles 40, and light-transmissive yellow particles 45 and whiteparticles 40, respectively, stand dispersed. A red reflection layer 61,a green reflection layer 62 and a blue reflection layer 63 are alsodisposed on the backs of the first cell, second cell and third cell,respectively. In this embodiment, there is such an advantage that thelight-screening performance of black display and the brightness andsharpness of color display are improved.

A color display method in the pixel structure shown in FIG. 13 isdescribed below with reference to FIGS. 14A to 14D, regarding respectivecases of white display, monochrome display, complementary-color displayand black display.

In the case of white display, as shown in FIG. 14A, in all cells, whiteparticles are arranged on the display electrodes, and color particlesare collected at the second collection electrodes. White incident lightis scattered at white particle layers, and white light is emittedwithout being modulated.

As an example of monochrome display such as red display, green displayor blue display, a case of green display is shown in FIG. 14B. In thefirst cell, white particles are collected at the first collectionelectrode, and on the other hand, a cyan particle layer is formed on thedisplay electrode. In the second cell, white particles and magentaparticles are collected at the first collection electrode and secondcollection electrode, respectively, to make the green reflection layeruncovered. Also, in the third cell, white particles are collected at thefirst collection electrode, and on the other hand, a yellow particlelayer is formed on the display electrode. White incident light ismodulated to green light as a result that it is all absorbed by thesubtractive color mixing between the color particle layers and the colorreflection layers and the green light component is reflected in thesecond cell, and is emitted.

As an example of complementary-color display such as cyan display,magenta display or yellow display, a case of magenta display is shown inFIG. 14C. In the first cell, white particles and cyan particles arecollected at the first collection electrode and second collectionelectrode, respectively, to make the red reflection layer uncovered. Inthe second cell, white particles are collected at the first collectionelectrode, and meanwhile a magenta particle layer is formed on thedisplay electrode. In the third cell, white particles and yellowparticles are collected at the first collection electrode and secondcollection electrode, respectively, to make the blue reflection layeruncovered. White incident light is all absorbed by the subtractive colormixing between the magenta particle layer and the green reflection layerin the second cell, and modulated to magenta light by the additive colormixing between the red light component scattered in the first cell andthe blue light component scattered in the third cell and is emitted.

In the case of black display, as shown in FIG. 14D, in the respectivecells, white particles are collected at the first collection electrodes,and meanwhile color particle layers are formed on the displayelectrodes. White incident light is all absorbed by the subtractivecolor mixing between color particle layers and color reflection layerswhich stand in a complementary-color relation to each other.

(Constituent Members and Their Formation Methods)

For the substrates, usable are plastic films formed of polyethyleneterephthalate (PET), polycarbonate (PC) or polyether sulfone (PES), aswell as quartz and glass. For the observation side substrate, atransparent material must be used. For the back substrate, however, acolored material such as polyimide (PI) may be used.

As electrode materials, any materials may be used as long as they areconductive materials capable of patterning. As display electrodematerials, usable are transparent electrode materials includinginorganic materials such as indium-tin oxide (ITO), organic materialssuch as PEDOT (trade name; available from AGFA Co.). As collectionelectrode materials and back substrate side display electrode materials,usable are, e.g., metals such as chromium (Cr), titanium (Ti), aluminum(Al) and copper (Cu), carbon, and silver paste, as well as organicconductive films. Where the display electrode on the back substrate sideis used also as a light reflection layer, a material with a highreflectance such as silver (Ag) or aluminum (Al) may preferably be used.Where this display electrode is used as a white display electrode, theelectrode surface itself is made to have surface unevenness so that thelight may reflect irregularly, or a light-scattering layer is beforehandformed on the electrode.

As materials for the insulating layer, usable are materials which arethin-film and can not easily form pinholes and have a low dielectricconstant, as exemplified by amorphous fluorine resins, highlytransparent polyimide resins, PET, acrylic resins and epoxy resins. Theinsulating layer may preferably have a layer thickness of approximatelyfrom 10 nm to 1 μm.

As materials for the partition walls, polymer resins may be used. Thepartition walls may be formed by any methods, for example, a method inwhich a photosensitive resin layer formed of acrylic resin or the likeis formed on the substrate by coating, followed by exposure andwet-process development; a method in which partition walls separatelyformed are bonded to the substrate; a method in which partition wallsare formed by printing; and a method in which partition walls arepreviously formed on the surface of a light-transmissive first substrateby molding. Where the partition walls themselves are made up of aconductive material to serve as collection electrodes, electrolyticplating, resin molding or the like may be used. As methods of forminginsulating layers on such conductive partition wall surfaces, thefollowing may be used: for example, a method in which the electrodesurfaces are oxidized by anodization, and a method in which theelectrode surfaces are coated with a resin by electrodeposition resistcoating.

As the insulating liquid (migration liquid for electrophoresis), usableare aromatic hydrocarbons such as benzene, toluene, xylene, andnaphthene type hydrocarbons; aliphatic hydrocarbons such as hexane,cyclohexane, kerosene, paraffin type hydrocarbons and isoparaffin typehydrocarbons; and halogenated hydrocarbons such as chloroform,trichloroethylene, dichloromethane, trichlorotrifluoroethylene andbromoethyl; as well as silicone oil and high-purity petroleum. Aninsulating liquid having a different specific gravity may also be addedin order to adapt specific gravity to that of charged particles.

For the charged particles used in the present invention, any ofinorganic materials, polymeric materials and composite particles ofthese may be used without any particular limitations as long as thescope of the present invention is satisfied. In the case when polymericmaterials are used, the following may be used, but not limited to, forexample, polyacrylic resins such as polyacrylate resins,polymethacrylate resins, ethylene-acrylic acid copolymer resins, phenolnovolak type epoxy resins, cresol novolak type epoxy resins,cycloaliphatic epoxy resins, glycidyl ester epoxy resins, and glycidylphthalate epoxy resins. The charged particles may preferably have a sizeof from 0.05 μm to 10 μm in particle diameter. The charge polarity ofthe charged particles is controlled by particle constituent materialsand/or charge control agent modified on particle surfaces.

The colored charged particles may preferably be colored with a dye.Where white charged particles are used, titanium oxide may be used, and,where black charged particles are used, pigments such as carbon black,Nigrosine and black iron oxide may be used. Light-transmissive coloredcharged particles may preferably be those having been colored with adye. As the dye, there may be preferably used oil-soluble dyes such asazo dyes, anthraquinone dyes, quinoline dyes, nitro dyes, nitroso dyes,perinone dyes, phthalocyanine dyes, metal complex salt dyes, naphtholdyes, benzoquinone dyes, cyanine dyes, indigo dyes and quinoneiminedyes. Any of these may also be used in combination.

The dye may specifically include, e.g., Varifast Yellow 1101, 1105,3108, 4120; Oil Yellow 105, 107, 129, 3G, GGS; Varifast Red 1306, 1355,2303, 3304, 3306, 3320; Oil Pink 312; Oil Scarlet 308; Oil Violet 730;Varifast Blue 1501, 1603, 1605, 1607, 2606, 2610, 3405; Oil Blue 2N,BOS, 613; Macrolex Blue RR; Sumiplast Green G; and Oil Green 520, BG.

As the charge control agent added to the insulating liquid, usable areanionic surface-active agents, cationic surface-active agents,amphoteric surface-active agents, metallic soaps, nonionicsurface-active agents, fluorine type surface-active agents, block typecopolymers, and graft type copolymers, any of which may be used alone orin the form of a mixture of two or more, including sulfonated oils,alkyl phosphoric esters, and succinimides. These may each be addedalone, or may be added in a combination of two or more. Specificexamples may include cobalt naphthenate, zirconium naphthenate,zirconium octenoate, calcium petronate, lecithin, and OLOA 1200(available from Chevron Corp.).

EXAMPLE

The cell structure, cell fabrication process and driving method aredescribed below in greater detail by giving Example.

In this Example, there is described a color electrophoretic displaydevice in which pixels so structured that three unit cells are disposedin parallel to constitute each pixel are arranged in a matrix form. Thedisplay device to be fabricated have 100×100 pixels, and each pixel hasa size of 300 μm×300 μm. FIGS. 15A and 15B present a cross-sectionalview (FIG. 15A) of areas corresponding to 2×2 pixels which are part ofthe display device, and a plan view (FIG. 15B) along the line 15B-15B inFIG. 15A.

The pixel structure in this Example is characterized in that partitionwalls themselves serve as collection electrodes, and is grouped into thestructure described with reference to FIGS. 5A and 5B. Each pixel isconstituted of three unit cells of 100 μm in width and 300 μm in lengthwhich are disposed in parallel. The charged particles arelight-transmissive polymer particles colored with desired dyes. A firstcell (left side), a second cell (middle) and a third cell (right side)are filled with insulating liquids in which cyan positively chargedparticles and red negatively charged particles, magenta positivelycharged particles and green negatively charged particles, and yellowpositively charged particles and blue negatively charged particles,respectively, stand dispersed. The particles all have an averageparticle diameter of 1 to 2 μm.

The substrates consist of a first substrate 101 serving as theobservation side substrate and a second substrate 102 serving as theback substrate. The pixels are each square in planar shape. At themiddle area of each unit cell, a display electrode 2 is disposed on theback substrate (FIG. 15A). The respective unit cells are divided byconductive partition walls 31 and 32 which function as collectionelectrodes. Conductive partition walls facing each other along the unitcell lengthwise direction and with the display-electrodes positionedtherebetween function as first collection electrodes 31 and secondcollection electrodes 32.

In FIG. 15A, reference numeral 9 denotes an insulating layer; and 10denotes a scattering layer.

The respective collection electrodes need not be insulated for eachpixel. In this Example, the collection electrode partition walls formlines connected between unit cells in the unit cell lengthwisedirection, and lines of collection electrodes insulated from one anotherare alternately arranged in the unit cell width direction. Meanwhile,the respective display electrodes are insulated from one another, andtheir potentials are independently controlled by a switching elementconnected for each display electrode. In this Example, switchingelements 72 connected to the display electrodes 2 through contact holes14 are disposed at the pixel boundary regions on the surface of the backsubstrate 102 (FIG. 15A). The respective switching elements are FET typethin-film transistors (TFTs), and control the potentials of displayelectrodes connected to drain electrodes by applying a desired voltageto signal lines connected to source electrodes and to scanning linesconnected to gate electrodes.

A fabrication process for the electrophoretic display device accordingto this Example is described below with reference to FIGS. 16A to 16C.

First, using a glass substrate of 1.1 mm in thickness, a back substrate102 is fabricated (FIG. 16A). First, scanning electrode lines and gateelectrodes are formed using Cr, then an SiN film is formed on the wholesurface, thereafter a-Si layer/n⁺layer are formed to provide switchingelement areas, and then source electrodes, drain electrodes, signalelectrode lines and first common electrode lines (all not shown) aresuccessively formed using Al, followed by patterning to form theswitching elements 7, which are bottom gate type FETs. Then, these arecovered with an insulating layer (not shown), and thereafter contactholes 14 are made in the insulating layer. On this insulating layer, thedisplay electrodes 2 and second common electrode lines (not shown) areformed in thin films of ITO. The display electrodes 2 are connected tothe drain electrodes of the switching elements 7 through the contactholes 14. Subsequently, these are covered with a resin insulating layer3, and then further contact holes (not shown) are formed at cross pointsof first common electrode lines and pixel boundaries and at cross pointsof second common electrode lines and pixel boundaries. Next, on thisinsulating layer 3, an electrode film for plating (not shown) is formed,and a thick-film resist pattern (not shown) is formed thereon.

Then, electrolytic plating is effected on the areas where the electrodefilm for plating stand uncovered, to form first collection electrodes 31and second collection electrodes 32 which both serve also as partitionwalls. In the course of this step, the respective collection electrodesare connected to the first common electrode lines or the second commonelectrode lines through their contact holes. Next, the thick-film resistpattern is removed by dissolving it, and subsequently the electrodesformed by plating thus uncovered and the surfaces of the respectivecollection electrodes are anodized in an aqueous oxalic acid solution tomake insulative and transparent the electrodes formed by plating andalso form insulating layers (not shown) on the surfaces of therespective collection electrodes.

Next, the cells formed on the back substrate 102 are filled withinsulating liquids 5 in which charged particles have been dispersed. Forthe insulating liquids, isoparaffin (trade name: ISOPER; available fromExxson Chemical Co.) is used. In this Example, liquids in whichdifferent types of particles have been dispersed must be put in the unitcells adjoining to one another. Accordingly, three types of droplets 16of the liquids each containing particles for one unit cell aresuccessively injected through nozzles 16 by means of an ink jet devicehaving a multi-nozzle (three nozzles). When injected, a voltage isapplied across the first collection electrodes 31 and the secondcollection electrodes 32 so that the particles injected are collected atthe collection electrodes immediately after they have been shot in, toprevent the particles from being transported to pixels to pixels (FIG.16B).

Subsequently, in the state that the voltage is applied across thecollection electrodes, the observation side substrate 101 is disposed onthe top faces of the partition walls formed on the back substrate 102.(In the observation side substrate 101, used is a glass substrate of 0.5mm in thickness which has been covered on its surface with an insulatinglayer 9.) In this state, the observation side substrate 101 is, withheating, uniformly pressed against the top faces of the partition wallsto join them with an adhesive. Thereafter, the peripheries of theobservation side substrate 101 and back substrate 102 are sealed. Thusthe electrophoretic display device is completed (FIG. 16C).

The electrophoretic display device thus fabricated was connected to adriving device (not shown) to inspect display operation in the followingway.

First, as whole-area initial reset operation, selection signals areapplied to all the scanning lines to set the gates of all the pixels ON,in the state of which 0 V is applied to all the signal lines, and at thesame time 300 ms rectangular pulses of −20 V and +20V are applied to thefirst collection electrodes and the third collection electrodes,respectively, to make the cyan particles, the magenta particles and theyellow particles collect to the first collection electrodes, and the redparticles, the green particles and the blue particles to the secondcollection electrodes. After the pulses have been applied, non-selectionsignals are applied to all the scanning lines to set the gates of allthe pixels OFF to complete the initialization reset operation. In thestate of initial reset, the whole area stands white.

Writing operation is made by applying selection signals to the scanninglines in order in the same manner as in usual active matrix drive, andapplying to the selected signal lines the writing signals correspondingto the scanning lines selected in synchronization with selectionperiods. Writing signals for writing desired colors are applied inrespect of all the cases of white display, monochrome display,complementary-color display and black display.

In the case of white display, 0 V is applied as writing signals to thedisplay electrodes to retain the reset state.

In the case of green display, writing signals of +20 V are applied tothe first cells to make the cyan particles move to the displayelectrodes, writing signals of −20 V are applied to the second cells tomake the green particles move to the display electrodes, and writingsignals of +20 V are applied to the third cells to make the greenparticles move to the display electrodes.

In the case of magenta display, writing signals of −20 V are applied tothe first cells to make the red particles move to the displayelectrodes, writing signals of +20 V are applied to the second cells tomake the magenta particles move to the display electrodes, and writingsignals of −20 V are applied to the third cells to make the blueparticles move to the display electrodes.

In the case of black display, writing signals of −20 V are applied tothe first cells to make the red particles move to the displayelectrodes, writing signals of −20 V are applied to the second cells tomake the green particles move to the display electrodes, and writingsignals of −20 V are applied to the third cells to make the blueparticles move to the display electrodes.

Color display images obtained by the above method were bright and sharp,bringing the effect as expected.

1. An electrophoretic display device comprising a substrate and unit cells provided thereon, each of which comprising: two types of charged particles which are different from each other in charge polarity and coloration; one display electrode forming a display area, two collection electrodes for collecting said two types of charged particles; and a driving means capable of forming a state in which each of the two types of charged particles have been collected at each of the collection electrodes, a state in which one of the two types of charged particles have been disposed on the display electrode and the other type of charged particles have been collected at one of the collection electrodes, a state in which said one of the two types of charged particles have been collected at one of the collection electrodes and said other type of charged particles have been disposed on the display electrode, and a state intermediate between these states, wherein said unit cells are arranged in matrix form, and each of the two collection electrodes in each unit cell is a common electrode to which a voltage common to all unit cells is to be applied, and wherein the display electrode in each unit cell is an electrode which is driven individually.
 2. The electrophoretic display device according to claim 1, wherein said two collection electrodes are disposed at a position where they lie substantially one upon another as viewed from the observer side.
 3. The electrophoretic display device according to claim 2, wherein said unit cell has a partition wall which divides adjoining unit cells, and said two collection electrodes are disposed on both ends of the partition wall or in the interior of the partition wall. 